1. Field of the Invention
The present invention relates to a perforating apparatus for forming a pair of row groups of perforations in a web to be reduced to tip papers used in the manufacture of filter cigarettes, for example.
2. Description of the Related Art
Recently, many smokers have started to love filter cigarettes of relatively light taste. Commercially available, therefore, are filter cigarettes in which a number of perforations are formed in the tip paper. If the perforations are bored through the tip paper, cigarette smoke can be thinned by air which flows into the mouth through the perforations and the filter, so that tar and nicotine in tee smoke can be satisfactorily adsorbed by the filter.
The perforations can be formed in the tip paper by two methods. In one method, a number of perforations are bored through the tip paper of a filter cigarette after the cigarette is finished. In the other method, a great number of perforations are previously bored through a web to be reduced to tip papers, before a cigarette and its filter are connected by means of the tip paper.
Perforating apparatuses for effecting the aforesaid second method are disclosed in U.S. Pat. Nos. 4,302,654 and 4,507,535, for example. Both these apparatuses are provided with a hollow cylindrical mask member which is supported for rotation. A number of openings are formed circumferentially at regular intervals in the peripheral surface of the mask member.
A web is transported in a manner such that it is wound on part of the peripheral surface of the mask member, while causing the mask member to rotate. Laser beams are applied to the web through the openings of the mask member, in the course of the transportation of the web, so that a row or rows of perforations are formed in the web.
The aforementioned conventional perforating apparatuses are suited for the formation of one or two rows of perforations. For the formation of three or more rows of perforations, however, the internal construction in the mask member must be complicated. More specifically, in the conventional perforating apparatuses, the mask member must contain beam guides as many as the rows of perforations to be bored through the web. Thus, these apparatuses are unfit for the formation of a relatively large number of rows of perforations.
In the conventional perforating apparatuses, moreover, rows of openings as many as the rows of perforations to be bored through the web must be formed in the mask member. If the rows of openings are thus independent from one another, it is difficult to make uniform the shape and array of the perforations formed by means of the laser beams applied through the individual rows of openings of the mask member. Accordingly, the external appearance of finished products may possibly be spoiled.